Custer feldspar vs Mahavir potassium feldspar

[Clay Rabbit]

Has anyone had issues changing to Mahavir potash feldspar from the now unattainable Custer feldspar?  I have noticed is makes my glaze thicker. About to fire a glaze test.

[Clay Rabbit]

It's to cone 10.

[Mark C.]

Not many cone 10 folks in these parts. I am one of them who fire soft cone 11 with my porcealain. I stocked up with a 1/2 ton of custar so I will maybe make it thru rest of potter life without finding out what the Mahavir  will do. Took me 40 years to use up my 3,000# of Kingman feldspar last year. Thicker sounds like more water to get your specific gravity right. Digital fire can answer what the differences may be

[Hulk]

Min posted a comparison: Potash Feldspar comparisons of Custer, Mahavir, G200EU and Vardhman - Clay and Glaze Chemistry - Ceramic Arts Daily Community

There's some detail/history in this thread: Custer Feldspar Substitution - Clay and Glaze Chemistry - Ceramic Arts Daily Community

...check back for more input.

[Min]

Hi @Clay Rabbit and welcome to the forum.

I think that as time goes by and more people have to start using alternatives to Custer Spar we will get a better feel for how it behaves in the bucket and fired onto pots compared to Custer.  Given a cone 10 glaze is going to have more spar in it than the typical cone 6 glaze any differences between the two spars is more likely to show up with the high fire. SG the same with both recipes? Old supply of Custer or within the past few years? A one to one swap in your recipe?

Formula speaking the analysis for Custer 06/21 compared to Mahavir (analysis date unknown from Laguna) are pretty close for alumina and silica. K2O is higher and Na2O lower with the latter. I haven't had to swap over to Mahavir yet so I can't comment if it melts better or worse but just by looking at the data it should be about the same.